Relaying is considered for LTE Advanced as a tool to improve, for example, the coverage of high data rates for User Equipment (UE), temporary network deployment, the cell edge throughput and/or to provide coverage in new cell areas. LTE Advanced supports relaying by having a Relay Node (RN) wirelessly connected to a base station (eNB) (referred to as a Donor eNB (DeNB)). In addition to serving its own ‘donor’ cell, the Donor eNB serves the RN, via a modified version of the Evolved Universal Terrestrial Radio Access (E-UTRA) radio interface. The modified interface is referred to as the ‘Un’ interface.
Each RN is provided with many aspects of a base station's functionality and is therefore capable of acting as a base station serving its own ‘relay’ cell. From the perspective of the user equipment in the relay cell, therefore, the RN essentially appears to be a conventional LTE base station. In addition to the base station functionality, however, the RN also supports a subset of the UE functionality including, for example, many aspects of the physical layer, Medium Access Control (MAC), radio resource control (RRC), and non access stratum (NAS) functionality, to allow it to connect wirelessly to a Donor eNB.
In some situations, the RN will be installed in a moving vehicle, such as on a train, bus, ship or the like. Such a mobile RN will change its attachment from one Donor eNB to another, as its geographical location changes. When a mobile RN moves to a new Donor eNB that belongs to a new Tracking Area (TA), the mobile RN will broadcast an identifier for the new TA to the UEs it serves. This will then cause these UEs to initiate signalling to the core network to inform the network of the new TA in which the UE is located (for the purpose of finding the UE when an incoming call is made to the UE). For UEs already actively connected to the core network, this involves sending an RRC connected mode mobility signalling message and for UEs in an Idle mode, this involves sending a TA update message to a Mobility Management Entity (MME) in the core network.
As these UEs will all change TAs at the same time, there will be a surge of signalling messages and this can result in signalling failure over the air interface between the RN and the Donor eNB and can result in a loss of connection for UEs in an active mode.
The inventors have realized that there is a need to provide a new way to inform the network about the change of tracking area of UEs connected via such a mobile RN.